Regional averages of the major element chemistry of ocean ridge basalts, corrected for low-pressure fractionation, correlate with regional averages of axial depth for the global system of ocean ridges, including hot spots, cold spots, and back arc basins, as well as “normal” ocean ridges. Quantitative consideration of the variations of each major element during melting of the mantle suggests that the global major element variations can be accounted for by ∼8–20% melting of the mantle at associated mean pressures of 5–16 kbar. The lowest extents of melting occur at shallowest depths in the mantle and are associated with the deepest ocean ridges. Calculated mean primary magmas show a range in composition from 10 to 15 wt % MgO, and the primary magma compositions correlate with depth. Data for Sm, Yb, Sc, and Ni are consistent with the major elements, but highly incompatible elements show more complicated behavior. In addition, some hot spots have anomalous chemistry, suggesting major element heterogeneity. Thermal modeling of mantle ascending adiabatically beneath the ridge is consistent with the chemical data and melting calculations, provided the melt is tapped from throughout the ascending mantle column. The thermal modeling independently predicts the observed relationships among basalt chemistry, ridge depth, and crustal thickness resulting from temperature variations in the mantle. Beneath the shallowest and deepest ridge axes, temperature differences of approximately 250°C in the subsolidus mantle are required to account for the global systematics.

/pdf/global-correlations-of-ocean-ridge-basalt-chemistry-with-37kk1l47q8.pdf

Global correlations of ocean ridge basalt chemistry with axial depth and crustal thickness

The role of water in the petrogenesis of Mariana trough magmas

The total geoid anomaly which is the result of a given density contrast in a convecting viscous earth is affected by the mass anomalies associated with the flow induced deformation of the upper surface and internal compositional boundaries, as well as by the density contrast itself is discussed. If the internal density contrasts can be estimated, the depth and variation of viscosity with depth of the convecting system can be constrained. The observed long wavelength geoid is highly correlated with that predicted by a density model for seismically active subducted slabs. The amplitude of the correlation is explained if the density contrasts associated with subduction extend into the lower mantle or if subducted slabs exceeding 350 km in thickness are piled up over horizontal distances of thousands of km at the base of the upper mantle. Mantle wide convection in a mantle that has a viscosity increasing with depth provides the explanation of the long-wavelength geoid anomalies over subduction zones.

/pdf/subducted-slabs-and-the-geoid-constraints-on-mantle-rheology-2k3td07kqh.pdf

Subducted slabs and the geoid: Constraints on mantle rheology and flow

http://www.mantleplumes.org/WebDocuments/Dick1984.pdf

Mineralogic variability of the uppermost mantle along mid-ocean ridges

A mechanism is proposed to explain epeirogenic motions of craton interiors in terms of the response of the lithosphere to subduction. The effects of changes in sea level are distinguished from subsidence of the basement by analyzing the tilt of chronostratigraphic sequences in which the bounding horizons were deposited at approximately the same elevation with respect to sea level. As an example, the Late Cretaceous subsidence and Tertiary uplift of the western interior of North America is examined, and a maximum tilt amplitude of 3 km, with a horizontal deflection scale of approximately 1400 km, is inferred. The link between platform sedimentation and subduction is tested by using numerical models of mantle convection which mimic the subduction and by examining the horizontal scale of the deflections to the overlying lithosphere. It is found that this scale is relatively insensitive to the temperature contrast between the slab and the surrounding mantle, the flexural rigidity of the lithosphere, and even the physical process assumed to govern the subduction. The most important factor affecting the scale is the dip of the subduction zone, and shallower subduction angles (less than 45°) can produce horizontal deflections of the order of 1000 km or more. In contrast, the vertical scale of the deflection is sensitive to all the above parameters. Using these results, two subduction models are introduced which predict both the time and length scales of the North American tilt, and it is conjectured that the process may be responsible for other regions of platform subsidence where subducted lithosphere may have passed beneath the continent at a shallow angle.

Tilting of continental interiors by the dynamical effects of subduction

It is shown that gravity and topography anomalies on the earth's surface may provide new information about deep processes occurring in the earth, such as those associated with mantle convection. Two main reasons are cited for this. The first is the steady improvement that has occurred in the resolution of the long wavelength gravity field, particularly in the wavelength range of a few hundred to a few thousand km, mainly due to increased coverage of terrestrial gravity measurements and the development of radar altimeters in orbiting satellites. The second reason is the large number of numerical and laboratory experiments of convection in the earth, including some with deformable upper and lower boundaries and temperature-dependent viscosity. The oceans are thought to hold the most promise for determining long wavelength gravity and topography anomalies, since their evolution has been relatively simple in comparison with that of the continents. It is also shown that good correlation between long wavelength gravity and topography anomalies exists over some portions of the ocean floor

and A B Watts

Papers

Long Wavelength Gravity and Topography Anomalies